Render Blender's object with size defined in pixels
I don't know if I get what you want, but this setup seems to get something like that: I put some colored cubes (each has one of three shadeless materials) on the diagonal of a a 10px x 10 px square.



The camera is ortho and its orthographic scale matches the output size. Antialiasing is obviously off.

This is the output (tiny 10px by 10px image):



setting resolution to 100x 100 and ortho scale to 100 you get this somewhat bigger image, but still each BU is a pixel:



Obviously you can change the aspect ratio, but keep the ortho scale equal to the bigger X/Y pixel value, like this:



Maybe this can work for you...